Aircraft-mounted systems for measuring the weights and the center of gravity of aircraft are known and are based on the principle of measuring the deformation of the axles and/or the wheel levers of the aircraft landing gear by means of relative displacement sensors, and on using an on-board computer to interpret the results of said measurements.
For example, U.S. Pat. No. 4 269 070 shows a type of relative displacement sensor together with its mounting on a pair of radial lugs, and the sensor itself may be of the inductive, capacitive, optical, piezo-resistive, . . . type. Each lug has at least one plane fixing face, and generally has two opposite plane fixing faces, with a sensor base being firmly fixed to the or each of said fixing faces. Each sensor component mounted on the two lugs of a pair extends towards the other in order to put its measuring components in an accurate position relative to one another. The deformation of the axle or the wheel lever which is to be measured causes the lugs to be displaced relative to each other and consequently it causes a relative displacement of the complementary measuring components.
Heretofore, each lug has had a round smooth hole passing through it between its opposite fixing faces. The sensor bases also have round holes passing therethrough and the sensor is fixed to each lug by means of bolts passing through these holes when in alignment.
Also French Pat. No. 80 16 046 and U.S. Pat. No. 4,472,976 show that in order to obtain accurate measurements, the measuring axis of the sensor must be positioned in the manner defined in said patents.
Over the last ten years, and in spite of there being no international standard on the subject, most of the axles and wheel levers of the landing gear in large aircraft such as those built by Boeing, Douglas, Airbus, etc. have been provided with pairs of lugs as defined above in order to enable relative displacement sensors to be fixed thereto.
The studies summarized below of the entire weighing and centering system explain and justify the need to use devices in accordance with the present invention for fixing two sensors to lugs of the type having two parallel bearing faces and not more than one smooth round hole perpendicular to said faces and substantially in the middle thereof:
firstly, it must be understood that in a weighing and centering system, the useful signal from each sensor as taken into account by the computer is the difference between the output signal from the sensor at a given moment and the signal which the sensor provides when the landing gear is not standing on the ground (which signal is referred to as sensor zero);
studies on the elastic deformation of the various axles and wheel levers of the aircraft concerned shows that the usable displacement of the sensor (corresponding to the useful signal applied to the computer) lies in the range 0.01 mm to 0.03 mm;
the maximum error which users are prepared to tolerate on weighing and centering is about 1%;
given the surface state of the lug bearing faces, which faces are protected by anticorrosion paint if a sensor fixed thereagainst is removed and then replaced, the new measurement obtained may differ from the preceding measurement by 0.005 mm to 0.02 mm, and this difference may be considerably increased if a sensor is replaced by a different sensor;
if a single sensor is fixed to each pair of lugs, the system ceases to work in the event of the sensor breaking down, and given the above remark concerning the effects of changing a sensor, the entire system needs re-setting if ever a sensor is replaced; and
if two sensors are fixed to each pair of lugs and if two computers are used, a back-up or redundant system is obtained in which the problem of a breakdown in one sensor appears to be solved since the other sensor mounted on the same pair of lugs can replace it.
However, when the faulty sensor is replaced, there arises a problem which the present invention seeks to solve. When a faulty sensor is replaced, the effect of releasing the other sensor from a lug is equivalent to the effect of replacing that other sensor, and from the above explanations it can be seen that the zero point of both sensors will need to be reset after the system has been reassembled.
Resetting the zero point would not be difficult if the aircraft could easily be weighed on scales or a weighbridge. However, this is not easily done for aircraft, and in particular for large aircraft.
With large aircraft the remaining solutions are: to trust the manifest which may be in error; to fly the aircraft empty; or else to jack the aircraft up so that no weight is applied to its landing gear with both of the "no-weight" solutions being long and expensive.
A cheap and effective solution consists in using a device in accordance with the present invention.
In order to improve the reliability of the measuring means, it is desirable to mount two sensors on each lug which are independent from each other so that when one of the sensors fixed to a lug needs to be removed or replaced, the other sensor fixed to the same lug is not disturbed.
The main aim of the invention is to provide a mounting device for mounting two sensors on a single lug which may have not more than one smooth round hole passing therethrough and which constitutes a part of an axle or a wheel lever in an aircraft, where it is desired to measure a particular deformation of the axle or wheel lever, said mounting device enabling two sensors to be fixed to said lug in a manner which allows each sensor to be totally independent of the other.